In the past, the achievement of transparency was pursued for the zinc oxide that is used as a UV protection agent in cosmetics. As a result, ultrafine particle powder with an average particle size of 100 nm or less has been mainly used.
The production of zinc oxide is mainly classified into a dry process and a wet process. As for the dry processes, these include the French process (indirect method) and the American process (direct method); in these processes, zinc oxide is produced by the oxidation of zinc vapor by air. The dry process is the main method for the production of ordinary zinc oxide. However, the wet process has been mainly used for the production of UV-shielding ultrafine particulate zinc oxide with a particle size of 100 nm or less.
The wet process is a method in which an aqueous solution of a zinc salt is neutralized with an alkaline agent, and the formed precipitate of zinc salt is washed with water, dried, and calcined to obtain zinc oxide. Generally, sodium carbonate is used as an alkaline agent to adjust the pH, and then the formed hydrozincite is decarboxylated by calcination at 300 to 500° C., thereby producing zinc oxide. Usually, zinc oxide produced by such a production method is called low-temperature calcined zinc oxide.
However, this method has a problem in that the formed fine particles of 100 nm or less aggregate due to sintering during calcination. As a result, the particle size increases, and the transparency and UV shielding property are undermined.
In order to suppress sintering of fine particles, zinc oxide whose particles are beforehand formed to various shapes has been proposed. For example, flaky zinc oxide powder is described in patent literature 1 and patent literature 2.
If the flaky powder is thin, however, the powder strength is not sufficient during the manufacturing of products such as cosmetics. If the thickness is increased to increase the strength, the particle size of the powder becomes too large, and an issue such as the scattering of visible light leading to a decrease in transparency arises in actual usage.
In patent literature 3, fine particle zinc oxide is supported inside the pores of porous spherical silica. However, when the zinc oxide with effective particle sizes for UV protection is supported, the amount supported will be too small. If the supported amount is increased, the smaller zinc oxide than the zinc oxide with effective particle sizes for UV protection will be supported; as a result, the effective UV protection ability cannot be achieved. When a large amount of sufficiently large zinc oxide is supported, the particle size of carrier silica will become too large; as a result, the efficiency of UV protection will become poor.
Other various approaches have also been tried, but they were hardly satisfactory.
Thus far, there is no reported synthesis method, of fine particle zinc oxide from zinc carbonate, wherein no calcining step is involved in order to prevent aggregation.    Patent Literature 1 Unexamined Japanese Patent Publication No. H01-230431    Patent Literature 2: Unexamined Japanese Patent Publication No. H06-115937    Patent Literature 1: Unexamined Japanese Patent Publication No. H07-291615